The invention relates to the field of control of direct current motors, and more particularly to a method of using sequential pulse width modulation for controlling and powering a plurality of direct current motors.
This invention relates generally to power and control systems for Direct Current [DC] motors. More specifically this invention relates to systems for individually powering and controlling a plurality of DC motors in both forward and reverse directions and through a continuum of speeds.
Direct Current motors have performed as the motive force in a variety of applications including locomotives, ships, trucks, vans, automobiles, farm equipment, forklifts, elevators, cranes, lawn mowers and trimmers, as well as leisure craft and vehicles, such as boats and golf carts. In select applications, such as locomotives, multiple direct current motors are used.
It is known in the prior art to control the speed of a direct current series motor by using a chopper circuit including a thyristor switch device in series with the motor. This is a more efficient form of control than using resistance control systems. The control of the speed of the motor is achieved by varying the width of voltage pulses supplied to the motor so that the resulting average power supplied to the motor establishes the operational speed of the motor. A thyristor is a type of diode with a controlling gate that allows current to pass through it when the gate is triggered. The thyristor is commutated by the current applied to it in the reverse direction. The thyristor thus effectively acts like a diode in that it only permits conduction in one direction, however, the point at which it starts conduction can be controlled by application of a control pulse to the gate. The chopper circuit thus controls the speed of the motor by switching the input voltage on and off depending on what output voltage is required; the longer the chopper is switched on, the higher the output voltage. The time that the chopper is switched on for is known as the on-time, while the ratio of on to off time is the mark to space ratio or chopper ratio.
A number of direct current motors are typically used to power a locomotive, for example, usually 4 or 6 DC motors are used, but sometimes 2 or 8. In the past, a single chopper has been used to control the speed of the DC motors. This has a number of disadvantages. For example, if one of the wheels is slipping, the chopper reduces power to all of the motors which risks further exacerbation of the problem.
There is a need for a more effective power control system, one that allows each motor amongst a plurality of DC motors to be controlled individually and separately.
The present invention provides a method of controlling power provided from a power source to a plurality of direct current traction motors comprising providing an individual chopper circuit for each traction motor. According to one aspect of the invention there is provided a method of controlling power provided from a direct current power source to a plurality of direct current traction motors comprising: a) determining the power requirement for each motor at each of a number of successive time intervals; b) determining the necessary effective voltage and pulse width to achieve the desired power for each motor; c) sequentially pulsing power to each motor for a duration necessary to achieve the power requirement at each time interval.
The present invention further provides an apparatus for controlling power provided from a direct current power source to a plurality of direct current traction motors comprising: a) means for determining the power requirement for each motor at each of a number of successive time intervals; b) means for determining the necessary voltage and pulse width to achieve the desired power for each motor; and c) means for sequentially pulsing power to each motor for a duration necessary to achieve said power requirement at each time interval.